Hello again, unfortunately while transmission is running fine, so far anyway, there is another issue, that bugs me.
The symptoms are following. When the car is left for long time (it cools down totally), it starts nicely but after start it is shaking and idle is unstable. When I engage gear and starts to move, it behaves like it is starving, so it surge (like it is dropping and again raising RPM few times a second). After few seconds, around 7, everything is fine.
Second symptom, much more serious in my mind, is when the car is left for some time. It cools down a little, so when I get in and start it, temp needle is one point below running temp (problem occur when the car is standing for around 2 hours. At 1 hour it is still fine and it starts fine). At this stage the car have serious problem starting. I have to hold the starter turning for over 5 seconds before it starts. Sometimes, probably depending on temperature, car even dies immediately after starting. But after it starts, engine runs fine. There are no fault codes stored, when I plug in OBDII reader.
The car have 60t miles on clock and there seems to be no problem with tensioners. At least there is no sound after start. I am mostly having relatively little amount of petrol in tank (mostly reserve is on), if that is relevant. But could be, although I think, it does that even with full tank.
It is XJ8, 1998 (early, before update), 3.2.
Any ideas? : -)
Thank you all for your replies in advance.

 

I would suspect a failing fuel pump or check valve. It appears that you are developing a vapor lock that the pump can not overcome. When the engine cools the vapor lock is gone.

 

Although I have never let my fuel tank get so low it shows reserve (maybe I should to see if the light works)
Have you checked the fuel filter and when was the last time you had spark plugs fitted? Also don't put too much faith in the Temperature Gauge (unless there is a RealGuage fitted)
Going by what you have said I would have thought a code would present though.

 

I was also afraid, it might be fuel pump, but there are thousand other options and when WOT it goes as it should, so I do not think it is a pump. At least I hope so. : -) But status update. : -)
I took out all spark plugs, check gap and make sure all are 0.51 inch , cleaned them (will buy new ones, but they take time to deliver), cleaned coils (they were little bit dirty). No effect. Cleaned MAF sensor, no effect. Took out whole throttle body and disassemble it, only to find out, that IAC is "inside" and you can not get to it. After putting it all in, is seems, that it may work fine. Can't say for sure now, I will try in morning. But I do have few other questions, if anyone know answers.
First, I had some slack (well, I thought I have slack, but actually someone screw that plastic think under gas, so there was not enough of pedal travel) so I set it according to OBDII scanner to 100% when fully depressed. But I read somewhere here, that when you do that, you might get limp mode. I actually have it set slightly over 100%. Meaning 100% are archived before depressing pedal fully down, so technically I have 105% or something like that. So, is that all right or is it problem?
Second, there was posted, that there is a relearn mode for TPS. Putting key into 1st position, depressing pedal down twice and then starting. I tried this to no effect on car behaviour on start. Nothing in workshop manual, so does this work or does something like this exist?
And one last thing, when the engine is off, I have throttle at 5% when pedal is not pressed. This goes to around 3% when engine is on at idle. Should it get to 0% under some circumstances? That all would go over IAC and shutter would be fully closed.
Btw. thank you all for your patience with me, I may be slightly paranoid, but I like when everything is working as it should. : -)

 

About spark plugs, they are probably original. They are supposed to last 100k miles, so they should be fine. But I will buy new ones and now they are cleaned. Temp gauge is of course good for nothing, but I was not using it for temp but as reference. I think engine does have same temp, when the needle is one below, no matter what temp that might be. Well, at least I hope so. : -)

 

Edit: I give this at the beginning, is it too long to read? I tried to put all relevant info in there, but it is rather long...

Ok, since no one answered I had to find it the harder way. : -) 100% is fine, relearn is in manuals (don't know for sure if it works, but well...) and TPS is variable by comp and never closes fully. Unfortunately my problems still persist. Well, at least that problem with starting after sitting for some time seems to be gone, for now. But I don't have enough relevant data to say that for sure.
But some issues still persist and I noticed another symptoms, which were there from the beginning but I did not notice. First, the car starts fine, but when cold, as the revs go down below 900 (after aprox 10-15 sec after start), whole car starts to shake violently, engine surges and according to OBDII reader spark advance changes from 5 to 13 degrees as the ECM is trying to cope with whatever is happening. This persist for around 15 seconds after which it suddenly disappear and will not appear until the car is again completely cold. But even after this, the idle is not as smooth as it should be and it is still shaking a little. Not much, but more, than it should, I think (sometimes the idle, when the car is at operating temp, is good and not shaking at all, but only sometimes). When I rev car above 1000, everything is fine and silky smooth.
Another issue is really bad fuel consumption. I get around 15mpg (16l/100km) with my very defensive style of driving (with chrysler 300m 3.5 from 2003 I get on the same road driving little bit faster around 24mpg), so that is definitely not normal.
So, what I am wandering. First problem cannot be caused by o2 sensors, as the car is in open loop. So is there some frequent problem with idle stepper motor? Maybe it gets stuck at some position and after more force is applied to it, as the revs drop, it overcome the resistance and all is fine. Or any ideas, what might cause that violent shake for relatively short amount of time? It does not seem to me like spark plug or coil problem, as it suddenly cure itself.
The other issue, of bad fuel consumption and slightly rough idle...when I connect in OBD reader, values are horrid, as long term bank 2 and 4 are around 20% (3 is at 25, which seems to be the limit of reader) and 1 and 3 and not much better (under 20%, but not much). (Anyone know which cylinders are bank 1, 2 3 and 4? I could not find that anywhere.) What is interesting, that 2 and 4 were in lower double digits recently and 1 and 3 in single digits (, but I unplugged O2 sensor connectors to measure it with ohm meter and after few tries, suddenly the short term fuel trim is in negative double digits for all banks. But long trim stays at high positive values, for some reason, and is actually much higher than before, as all banks are now at double digits. But the exhaust, when engine is at operating temp, is not smelling like rich. MAF sensor seems to be working fine (idle is at 4.5 g/s, goes up with throttle position and values are withing specs), intake temp is fine, coolant temp fine, plugs although not new are cleaned and gaps measured, coils cleaned from small amount of dirt. o2 sensors are giving, when at reader, relatively wrong values, maybe. According to manual, when they are warm and you rev engine to 2500, you should get voltage between 0.6V and 1V, which according to OBD is jumping from anything between 0V to 1V. But when I measured it with ohm meter, the were always above 0.6V. Also all other things (reving fast and dropping RPM fast) give the desired values. No error codes on OBDII. I will change spark plugs, but I still don't think they are the culprit. So, any ideas anyone? I am afraid that otherwise it will descent into change one thing and see, if that cures it. First spark plugs, than fuel filter, than o2 sensors, than coils,...which needles to say, will be quite costly approach. : -)

 

Before you start throwing parts at it you might try some fuel system/injector cleaner like BG44K. Maybe even pull a couple of injectors for inspection. This is something I have not messed with yet, but used to have to keep an eye on when I had a diesel aux. sailboat.

 

I'm also still thinking the check valve is allowing pressure to bleed off or the injectors are leaking. Have you checked fuel pressure vs. time after shutting engine off?

 

Well I actually did that last time I drove the car, but I think it is a wishful thinking, that this would solve the problem. : -) But it would be nice, no problem there. : -)

I have not checked pressure, as I am not sure I have the right equipment for it, but I will consult manual. That is good idea and can't hurt. But what do you mean vs time? The pressure should stay the same after shutting engine and if it start to go down, than the valve is not holding the pressure?
Is it by any chance located near the fourth cylinder on left bank if you are standing in front of car and looking under the hood, when fourth cylinder is the furthest from you? If yes, there was probably some leaking due to rubber hose being to big. But can't say that for sure and it is changed now anyway.

 

There is a schrader valve on the fuel rail, looks like a tire valve. When the engine has sat long enough to probably cause a starting problem, depress the valve stem and see if there is much fuel pressure. Keep a rag under it, of course.
There are fuel pressure test kits available in many places, but I have no idea what you can find. Check the internet for a kit, or maybe a local garage can help.
The pressure should hold after the engine is shut off. If it drops off, you could form a vapor lock in the fuel rail. A bad check valve or leaking injectors could cause this problem, hence my suggestion of watching the pressure vs. time.